This invention relates generally to magnetic resonance imaging (MRI), and more particularly to an MRI system construction that reduces noise generation related to creation of magnetic gradients in a static magnetic field.
Noise generation has always been a significant problem in MRI apparatuses where living subjects are imaged in the bore of an MRI magnet. High frequency switching of the currents in the gradient coils used in magnetic resonance imaging applications produces alternating Lorentz forces. Applied at high frequencies, resultant vibrations of the gradient coil assembly and any vibrations transmitted to the MRI static field magnet, produce acoustic pressure levels which can be harmful to the unprotected ears of the patient and operator. For fast acquisition of magnetic resonant imaging such as echo-planar imaging (EPI), with rise times in a range of 250-2000 micro seconds, the sound pressure level (SPL) can exceed 130 dB.
Generally in MRI systems, a static magnet, which may be a superconducting device, creates a substantially homogeneous static magnetic field within a region to be imaged. A cylindrical patient opening that accepts a patient to be examined is surrounded by the superconducting magnet that generates the homogeneous magnetic field. In recent developments, static field magnets are plate-like generally flat devices that oppose each other across the patient opening. Electromagnetic gradient coils are positioned near the static magnet structure. When the gradient coils are energized in predetermined sequences, the static homogeneous field in the imaging area is momentarily altered to produce a controlled gradient magnetic field in a selected direction. Generally, three gradient coils control three orthogonal coordinate directions.
The gradient coils are cylindrical when the static magnet coils are cylindrical. The gradient coils are plate like when the static magnet is formed of parallel plates. During the MRI measurements, requisite gradient magnetic fields are created by currents that are rapidly applied and removed from these gradient coils, respectively. Rapidly switching current in the gradient coils creates alternating forces, as stated, in a continuous string of acoustical noise bursts within the patient opening. Acoustic frequency is directly related to the switching frequency of the gradient coil currents.
Whereas the diagnostic advantages of using MRI techniques far outweigh the unpleasantness related to intense levels of noise, a quieter MRI apparatus is greatly to be desired for use with living subjects, both patients and operators.
Generally speaking, in accordance with the invention, a quieter MRI apparatus is provided. The invention includes an integrated MRI gradient coil/MRI static field magnet assembly which produces significantly lower noise when the gradient coil is excited in a conventional manner. The assembly includes a gradient coil assembly that is directly coupled to the inner cylinder of the cryostat vacuum container enclosing a cylindrical superconducting. MRI static field magnet.
In one embodiment, the gradient coil assembly is directly coupled throughout its entire axial length and circumferential area to the container of the MRI static field magnet. This coupling eliminates any air space between the concentric gradient coil assembly and the static field magnet. Such an air space can act as a resonant acoustic volume producing very high levels of noise when the gradient coil assembly is excited. The source of noise is eliminated by elimination of this air space.
Further, the integrated gradient coil assembly/static field magnet system assembly is stiffer than a conventional system where the gradient coil assembly is attached to the main magnet only at its two extreme axial ends. A stiffer system produces lower velocities of the switched gradient coil, which in turn produces lower noise levels in the patient opening of the gradient coil assembly and in the ambient environment. Mathematical analysis indicated, and actual tests verify, that elimination of the air space and the acoustic resonance volume between the gradient coils and the static field magnet assembly produces a significantly quieter system. The construction is implemented by shrink fitting the inner cylinder of the outer vacuum container of the MRI static field magnet directly onto the gradient coil assembly.
In an alternative embodiment in accordance with the invention, an annular space separates the gradient coil assembly from the MRI static field magnet assembly, and the gradient coil assembly is rigidly coupled to the inner cylinder of the MRI static field magnet assembly by discrete coupling rings. The annular space or chamber between the two assemblies is broken into smaller volumes or subchambers, which do not interconnect in the axial direction and thereby prevent axial propagation of noise generated in the chambers. Acoustic treatment of the surfaces within the chambers, including evacuation of that space, can reduce the amount of noise escaping to the ambient environment.
Accordingly, it is an object of the present invention to provide an improved gradient coil mounting for an MRI apparatus that operates at reduced noise levels.
It is a further object of the invention to provide an improved gradient coil mounting construction that is stiffer than a conventional gradient coil mounting.
It is yet another object of the invention to provide an improved gradient coil mounting construction that generates noise at higher acoustic mode resonant frequencies than a conventional gradient coil mounting with the same excitation.
Still other objects and advantages of the invention will be apparent from the specification. The invention accordingly comprises the features of construction, combinations of elements, arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.